Both Fcγ and complement receptors mediate transfer of immune complexes from erythrocytes to human macrophages under physiological flow conditionsin vitro

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Abstract
Abnormal clearance by the mononuclear phagocytic system of immune complexes (IC) is important in the pathogenesis of systemic lupus erythematosus (SLE). We have developed an in vitro model to investigate the cellular mechanisms involved in the transfer of soluble IC from erythrocytes to human macrophages under physiological flow conditions. In this assay, erythrocytes bearing fluorescently labelled IC are perfused over monolayers of human monocytes or monocyte-derived macrophages in a parallel-plate flow chamber, and transfer quantified using confocal microscopy and flow cytometry. Using aggregated human IgG as a model IC, we have been able to demonstrate transfer of IC from erythrocytes to macrophages. Blocking studies with specific neutralizing antibodies have shown that both complement and Fcγ receptors are required for IC transfer. Blockade of CR4 (αxβ2 integrin), FcγRIIa or FcγRIII reduced transfer, while anti-CR3 (αmβ2 integrin) had no effect. Blockade of CR3, FcγRIIa or FcγRIII also reduced the number of adhesive interactions between fluorescently labelled IC-bearing erythrocytes and macrophage monolayers. Taken together with the transfer data, this suggests differing roles for these receptors in the human IC transfer reaction that includes an adhesive function which facilitates IC processing by mononuclear phagocytes. Finally, a functional effect of the FcγRIIa R131/H131 polymorphism, important in susceptibility to SLE, has also been demonstrated using this model. Uptake of IgG2 but not IgG1-containing soluble IC was reduced by macrophages from individuals homozygous for the R131 allelic variant of the receptor.

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